In the air conditioning apparatus as a type of refrigeration apparatus, the use of a natural refrigerant that has minimal effect on the environment as the refrigerant charged in the refrigerant circuit has recently been studied. When carbon dioxide or another refrigerant having a low critical temperature is used as the natural refrigerant, a refrigeration cycle operation is performed in which the refrigerant pressure on the high-pressure side exceeds the critical pressure of the refrigerant.
A technique is known for enabling high operating efficiency in an air conditioning apparatus that performs a refrigeration cycle operation in which the high-pressure side attains a pressure that exceeds the critical pressure of the refrigerant. In this technique, the refrigerant pressure range on the high-pressure side whose coefficient of performance is near maximum is prescribed as the set value of the refrigerant pressure on the high-pressure side with respect to the refrigerant temperature at the outlet of a cooler, and the degree of opening or the like of a pressure reducing means is controlled so that the refrigerant pressure on the high-pressure side conforms to the set value (see Japanese Patent No. 3679323).